In the field of plating metal substrates, for example, particularly a process for plating with a precious metal such as gold, selectively controlling the deposition of such plating metal is a significant cost saving step, often representing the difference between a commercially feasible process and a mere laboratory phenomenon.
The known processes fall within two generally accepted categories: wet plating and dry plating. Electroplating is a wet plating process. The process is most commonly carried out by forming an electrical circuit between the metallic workpiece and a spaced electrode, all while located in a liquid bath containing the material to be plated out. Note U.S. Pat. No. 4,427,498 to Wagner and U.S. Pat. No. 4,534,843 to Johnson, et al.
These conventional electroplating processes can be quite costly and complicated, and can be hazardous. Waste treatment and disposal are also significant practical problems. Further complications ensue when conventional electroplating is used for selective depositing of the metal onto predetermined areas of the workpiece. Furthermore, recycling of expensive unused gold can lead to problems.
There are other conventional techniques for depositing a noble metal such as gold on a substrate. Evaporation of gold contacts is in principle a dry plating or deposition alternative to wet electroplating. It is not considered practical, however, because conventional methods require high vacuum, deposit unnecessary gold which requires extensive recycling, are not easy to turn on and off for spot evaporation without heavily caking a shutter. Furthermore, conventional boat evaporation of certain alloy compositions needed for wear resistance may be difficult. The shortcomings of this technology illustrate the need for a method of deposition, in only specifically defined areas, with well-controlled thickness and the ability to maintain stoichiometry of the plating alloy.
A related dry method of gold deposition is suggested by the widespread use of pressure bonding of gold wires to pads in microelectronics packaging. Although this works for wires, it is not apparent how to apply the required pressure to a gold layer up to 30.mu." in such a way as to effect transfer to the intended stock.
Published patents and articles disclose further wet processing technology which use a pulsed energy beam, such as a laser, for selective electroplating. Note, for example, U.S. Pat. No. 4,217,183 to Melcher, et al; U.S. Pat. No. 4,239,789 to Blum, et al; U.S. Pat. No. 4,349,583 to Kulynych, et al; U.S. Pat. No. 4,432,855 to Romankiw, et al; U.S. Pat. No. 4,497,692 to Gelchinski, et al; and U.S. Pat. No. 4,511,595 to Inoue, as well as the technical publication Laser Enhanced Electroplating and Maskless Pattern Generation by R. J. von Gutfield appearing in the Applied Physics Letters, 35 (9), 1 November 1979, page 651. For example, the disclosure of Kulynych, et al. teaches that laser thermal agitation of the dipole layer adjacent the substrate in wet plating can produce accelerated plating where the laser strikes. However, this is generally too slow for macroscopic devices, such as switch contacts, because of the inherent inability to transport plating ions to the intended spot fast enough, and does not avoid the use of wet plating solutions.
Another example of laser processing is taught by Ehrlich, Osgood, and Deutsch in U.S. Pat. Nos. 4,868,005 and 4,615,904. Laser assisted chemical vapor deposition (LCVD) as described therein operates by means of a local laser-heated spot on the substrate pyrolizing organometallic constituents of a carrier gas, or direct laser dissociation of the organometallic molecules above the intended spot for plating. Again, this is too slow for most macroscopic applications, because of problems transporting and decomposing the organometallic fast enough.
Further, in a related area of technology, as described in U.S. Pat. No. 4,281,030 to Silfvast, a recipient surface is preheated as by a laser source in a predetermined pattern to facilitate the attachment of flux flowing adjacent the recipient surface. Similar technology is disclosed in U.S. Pat. No. 3,650,796 to Jackson, et al. and in U.S. Pat. No. 4,042,066 to Engl, et al. These two patents disclose pasting a thermally decomposable metal onto a recipient surface which is pyrolyzed in a predetermined pattern for forming an essentially plated surface. Pyrolizing, however, is inconvenient since it always requires a paste, a wet process itself, and line resolution is less.
Another similar technology is described in the literature for producing a thin, transparent layer of uniform thickness on a recipient surface. Note U.S. Pat. No. 4,571,350 to Parker, as well as Vacuum Deposited Thin Films Using a Ruby Laser, by Smith, et al, found in Applied Optics, Volume 4, Number 1, January 1965, Page 147; Pulsed Laser Evaporated SnO.sub.2 layer in the Journal of Crystal Growth, 56 (1982) page 429, North-Holland Publishing Company; Thin Films Made with Lasers by Nancy Stauffer found in the MIT Report, July/August 1984, Volume XII Number 7, Page 1; and Deposition of Amorphous Carbon Films from Laser Produced Plasmas by Marquardt, et al. in Mat. Res. Soc. Symp. Proc. Volume 38, 1985 Materials Research Society. Such disclosures, however, do not allow for selectivity when plating on a recipient surface.
U.S. Pat. No. 3,560,258 to Brisbane, IBM Technical Disclosure Bulletin, Vol. 8, No. 2, July 1965 to Potts, and U.S. Defensive Publication T988-007 to Drew disclose transfer methods employed for different applications. While these last disclosures relate to a dry technique for depositing material on recipient surfaces as with laser, all describe evaporation of the material in a vacuum enclosure. But more importantly, such methods lack spatial selectivity, or controlled geometry.
In U.S. Pat. No. 4,752,455 to Mayer, there is disclosed a pulsed laser transfer technique for transferring a metal film on a glass, i.e., transparent substrate to a target area of an electrically conductive material adjacent thereto. That is, pulsed laser energy is directed through the transparent substrate onto the conductive film at a sufficient intensity and for a sufficient duration to rapidly vaporize the metal film. The target materials are driven by film vaporization energy and by the reaction thereof against the glass substrate onto the opposing or object surface of a second substrate. See also the report by J. Bohandy et al. in the J. Appl. Phys., Vol. 60, No. 4, Aug. 15, 1986, entitled, "Metal Deposition From a Supported Metal Film Using an Excimer Laser".
A shortcoming of the above processes is the inability to control the uniformity and structural integrity of the deposited metal film. From experimental work by the inventors hereof, it was determined that in following such processes, the resulting film deposits were always nodular or grainy, and often discontinuous in thinner films, as if they had broken up in a molten or vapor phase and had coalesced again into droplets, islands, or crystallites. The appearance of such films of gold was lighter in color than normal, and "dusty". If the transfer had been made at very high laser power density, and at atmospheric pressure, the gold film would have been black, due to its colloidal nature. Thick gold films may be reflowed by a second laser pulse to provide continuous rough polycrystalline coverage, but smooth continuous films comparable to evaporated or electroplated gold are not produced by the processes of Mayer or Bohandy et al. It is theorized that the problem may have to do with the high rate of arrival of the deposited atoms (ions) on the surface of the workpiece, and the corresponding high density of metal vapor just above the workpiece surface. As a consequence, there is coalescence above or on the surface of metal into droplets or thick islands. In other words, the reason for discontinuous films by such earlier methods may be due to a combination of effects of turbulence in the driving vapor, coupled with absence of mechanical strength in the molten gold (or very poor mechanical strength even if some of the gold remains solid), and tendency of molten or dense vapor gold on or above the substrate surface to form gold clusters rather than fully wetting the substrate. Contrast this to gold evaporation, for example. Gold evaporation produces smooth mirror surfaces, because the atoms arrive individually or in very small clusters in a random distribution on a cool substrate, and stick very close to where they hit. Similarly, electroplated atoms arrive and attach essentially at random. Although nucleation sites are involved in both processes, there are sufficiently many of them and the range of the gold is sufficiently limited that smooth films result.
The present invention avoids such shortcoming by the provision of including a highly absorptive polymer layer intermediate the transparent substrate and metal film to be transferred. That is, this invention relies upon a different physical interaction of the laser and the metal, effected by incorporation of a polymer layer to protect the metal and provide lateral strength during the violent transfer. While it is conceded that single-shot deposition of molten or vaporized gold pushed by a possibly turbulent expanding hot vapor will not produce films as smooth and continuous as evaporated films, the present invention nevertheless teaches a method to transfer smooth continuous films previously formed by conventional evaporation or sputtering, rolling, or other methods, to yield a smooth film, such that the original properties are retained. That is, this method relies on a strong polymer film to provide lateral strength to the thin gold, to protect the gold from hot vapor and direct laser heating, and to itself provide the material to be vaporized by the laser. The invention hereof, particularly the role of such polymer layer, will become apparent in the description which follows.